1. Field of the Invention
The present invention relates to heat-expandable synthetic resin components used to seal and create acoustic baffles in automobile body cavities or the like, and which are prepared with a UV photoinitiator in the resin and UV crosslinking of selected surface portions thereof, so that upon heating the crosslinked surface portions will expand at a slower rate and to a lesser extent than other portions of the component. This allows predetermined directional expansion of the component rather than uniform expansion in all directions, permitting bonding of the component only to selected cavity walls and/or more efficient filling of void spaces. In preferred forms, the synthetic resin system includes a grafted anhydride polymer, an epoxide additive which reacts with the anhydride polymer upon heating, a blowing agent capable of effecting expansion when the system is heated, a UV photoinitiator and a UV crosslinkable resin system such as an acrylate resin.
2. Description of the Prior Art
Automotive vehicle bodies have a variety of hollow posts, cavities, passages and similar areas which can transmit undesirable noises, air currents, moisture, dust and other airborne particulates unless sealed to prevent infiltration of such sounds or contaminants. U.S. Pat. No. 5,373,027 describes a dry, initially non-tacky, heat expandable sealant and baffle component which may be formed into a desired shape so that upon insertion into post, cavity or passage of the vehicle body, the component will undergo heat-induced expansion when the vehicle body is conveyed through a baking oven forming a part of the primer or paint curing step of the automobile manufacturing process.
In accordance with the ""027 patent, the composition for preparing the dry, expandable sealant and baffle product includes a major amount of an ethylene-xcex1,xcex2 ethylenically unsaturated carboxylic acid copolymer which has been partially neutralized with a metallic ion such as zinc, a minor amount of a blowing agent such as a modified azodicarbonamide and a minor amount of a low molecular weight resin tackifier which has the property of imparting tackiness to the outer surface of the component when the latter is raised to an elevated temperature and thereby expanded by the blowing agent.
The dry expandable sealant and baffle composition expands within the cavity of the automobile body when it is conveyed through a conventional primer or paint bake oven. Conventional ovens for baking or curing a finish applied to an automobile body typically operate at temperatures in the range of about 140xc2x0 C. to about 200xc2x0 C., depending upon the coating to be dried and cured. The automobile body usually transits a particular baking oven in a time period of from about 10 to 15 minutes to as much as 2 hours. To that end, prior expandable baffle compositions have been formulated to preferably undergo expansion at a temperature of about 135xc2x0 C. to about 185xc2x0 C. The dry, expandable sealant composition and product of the ""027 patent has enjoyed substantial commercial success throughout the automotive manufacturing field. Recent popularity of expanded-in-place sealant and baffle products for automotive applications has been accelerated by the ease of use of the baffle elements and the fact that each can be appropriately sized and configured to custom fit specific body cavity areas. Furthermore, the fact that baffle expansion can be accomplished in situ while an automobile is passing through a baking oven avoids the necessity of providing additional equipment and manufacturing operations for that specific purpose.
Most prior heat-expandable synthetic resins used in automotive cavity sealant and baffle products are designed to uniformly expand when sufficiently heated. While this characteristic is useful in many cases, there are situations where it is undesirable. For example, a given cavity geometry may have irregular contours or tight, restricted spaces which may be only incompletely filled using a uniform expansion synthetic resin system. Also, it sometimes occurs that, for optimum performance, specific cavity wall surfaces should not be contacted by the synthetic resin upon expansion thereof.
U.S. Pat. Nos. 5,979,902 and 5,931,474 describe cavity sealing articles made up of one or more electron beam radiation crosslinked inner synthetic resin bodies surrounded by a non-crosslinked outer shell portion. In use, the sealant article is placed within a cavity and heated, causing the crosslinked inner bodies to expand and force the outer shell portion into sealing contact with the cavity walls. The expansion characteristics of the sealing articles may be altered by selection of the shape of the inner crosslinked bodies. The sealing articles described in these patents are deficient because of the need to fully surround the inner crosslinked bodies with a shell of non-crosslinked material. This makes it more difficult to precisely control the direction and extent of heat expansion. Moreover, fabrication of sealant articles of this type can be relatively complex, particularly when plural spaced-apart inner crosslinked bodies are employed.
U.S. Pat. No. 4,203,815 discloses crosslinked and foamed resin sheets which are prepared by applying high-energy ionizing radiation to a foamed sheet to crosslink the sheet whereupon the sheet is heated for expansion purposes. Ionizing radiation permeates into the interior of the foamable sheet, so that crosslinking occurs over the entire thickness of the sheet. Accordingly, differential and/or direction-oriented expansion of the foamable material is not possible.
There is accordingly a need in the art for improved heat-expandable components adapted for use in automobile body cavities or the like which can be readily designed for controlled, direction-oriented expansion upon heating, and which avoid the use of inner crosslinked bodies surrounded by a non-crosslinked outer shell.
The present invention overcomes the problems outlined above and provides heat-expandable molded synthetic resin components which are designed to preferentially expand upon heating thereof in desired directions and to desired extents. Accordingly the components may be placed within an automobile body cavity or the like so that upon heating difficult to seal areas may be adequately filled or contacted with expanded synthetic resin material.
A component of the invention is in the form of a body presenting an outer surface and formed of a synthetic resin matrix, with at least a portion of the body surface being UV crosslinked to a depth of up to about 0.5 mm, while other body portions are not crosslinked (i.e., the other body portions are not crosslinked or are crosslinked only to a lesser extent than the primary crosslinked surface portions). Accordingly, when the component is heated to a temperature sufficient to effect expansion thereof, the UV crosslinked surface portion expands at a slower rate and to a lesser extent than the non-crosslinked body portions. In effect, the crosslinked surface portion serves as a carrier or temporary barrier section to direct the expanding synthetic resin material during the early stage of a heating cure cycle. This carrier or barrier section will normally be thermoplastic enough to slightly expand (e.g., 50-100%), while the other body portions will expand to a more significant extent.
In preferred forms, the synthetic resin matrix making up the heat-expandable component includes a quantity of a grafted anhydride polymer, an amount of a polymer additive containing a terminal epoxide group which reacts with at least a part of the anhydride polymer when the component is heated, a blowing agent, a minor amount of a photoinitiator and a crosslinkable resin system. The ingredients of the matrix are typically mixed and formed into pellets or the like, and the latter may be used in an injection molding machine to yield finished components of desired shape and size. Thereupon, one or more portions of the surface of the molded component are masked with a metal sheet or the like, and the masked component is subjected to UV light. The masked portion of the component is therefore not crosslinked or only crosslinked to a relatively minor extent, whereas the non-masked portions are crosslinked.
In use, the selectively surface crosslinked component is placed within an automobile body cavity and when the latter is passed through a curing oven as a part of the normal manufacturing process, the component is caused to expand. However, this expansion is non-uniform owing to the presence of the individual crosslinked and non-crosslinked portions of the component body.